The invention relates to a method for processing compressed image data for reducing blocking artefacts.
Modern methods of video communication and image data exchange use encoding methods to reduce the transfer and/or storage bandwidth usually necessary when storing and/or transferring image data digitally. Therefore, a variety of image data encoding methods has been developed. In typical encoding methods—such as processors called JPEG, MPEG or the like—it is common to decompose the images or frames to be stored or transferred into small sized blocks. After dividing the images or frames into blocks, an encoding transformation is applied to the blocks. For example, the discrete cosine transformation (DCT) may be used. As a result of the encoding transformation instead of a distinct number of pixels an equivalent number of transformation coefficients is obtained. The set of coefficients for each block is then quantized so as to reduce the amount of information describing the distinct blocks.
Generally, a certain amount of image information is lost during the process of quantization and thereby so-called blocking artefacts are generated after reconstruction of the digitized image by applying the inverse encoding transformation to the set of quantized transformation coefficients to reconstruct the encoded image. These blocking artefacts appear in the region of the boundaries of adjacent image blocks as discontinuities of brightness, contrast and/or colour.
To reduce the amount of loss of significant information and the effect of the blocking artefacts, a variety of different algorithms and methods has been suggested.
The method according to EP-0 526 885 A2 uses lowpass filtering and is an efficient method to reduce blocking artefacts. However, the suggested method is built up as a part of the decoder and therefore, specific information with respect to the encoding/decoding process is necessary.
U.S. Pat. No. 5,774,597 discloses a method for image compression and decompression using overlapping cosine transforms. The method suggests to remove blocking artefacts from a compressed and restored digitized image by modifying the particular coding process. Furtheron, the corresponding decoder has to invert the pre-processing step.
EP 0 771 507 B1 suggests a method and an apparatus for reducing image data compression noise. According to the suggested method both, the encoding and the decoding processes are modified. Additionally, the suggested method needs further information with respect to the coded data stream to reduce blocking artefacts.
Most prior art methods for reducing blocking artefacts have in common that they need to modify the coding and/or the encoding process. Therefore, the known methods are not applicable as pure post-processing methods to already coded data material.